A wave slaps the Delaware Bay shore and an American horseshoe crab tumbles out of the surf. It's early May.

The crab claws its way in a slow, clumsy advance over a mudflat, jerking along like a toy Army tank with a cranky spring-wound motor.

One by one the crabs leave the sea for beaches on both sides of the bay until millions of these olive drab creatures blot mud and sand. It is one of the oldest journeys on earth, older than dinosaurs, and it is an important one. For it is on the bay's beaches that the horseshoe crabs breed, propagating a species that dates back 350 million years.

At water's edge males court females, attaching themselves to the rear of the female's carapace (or shell) with pincer-like appendages. Then the females tow their suitors up the beach, scratch out hollows in the sand, and lay tiny pea green eggs which are fertilized by the males. But the waves at high tide wash away much of the sand and soon billions of the eggs lay exposed on the beaches.

Thousands of miles away another biological clock is ticking. Red knots, ruddy turnstones, sanderlings, and semi-palmated sandpipers are already in flight, leaving behind their wintering grounds in Central and South America -- the mudflats of Surinam, the rocky nooks at Tierra del Fuego, the meadows on the Argentine Pampas.

They're winging some 7,000 miles towards the bay and the little green eggs which are now crucial to their survival. Depleted of fat reserves on arrival, many birds will almost double their body weight during their two-week stopover before departing on the next leg of their journey -- a 2,000-mile, non-stop flight to their Arctic breeding grounds. By late June, the shorebirds will be nesting on the thawing tundra.

The Delaware Bay is the principal breeding grounds for American horseshoe crabs on the East Coast and among the largest staging areas for shorebirds in North America. And it is unique in that there's only one main course on the menu: the little green eggs. Destroy the horseshoe crab's habitat and a vital link in the migratory chain would be broken, and thousands, perhaps millions, of shorebirds endangered.

Quick Facts About The Horseshoe Crab (Limulus polyphemus)

Although appearing crab-like, horseshoe crabs are more closely related to spiders. They are one of the Earth’s oldest creatures, having appeared here over 300 million years ago.

The horseshoe crab’s blue blood is used to test intravenous drugs, vaccines, and medical devices for bacterial contamination to protect public health. Studies on the eyes of horseshoe crabs have helped us learn about the function of human eyes.